1. Field of the Invention
The present invention relates to a conductive paste containing silver as the main component. Particularly, it relates to a conductive paste which is printed on a substrate and fired to form an electric conductor.
2. Discussion of Background
As a method for preventing fogging of a rear window glass of an automobile, a method has been employed wherein a conductive paste is printed on the glass surface to form a plurality of conductive strips for heating and bus bars connected to both ends of the strips and then fired to form an electric conductor, and metallic terminals for lead wires are attached to the bus bars by soldering, and a voltage is applied across the metallic terminals, so that the surface temperature of the glass is maintained at a level higher than the dew point by heat generation of the conductor.
In this case, a constant voltage power source is used, and the heating value is accordingly governed by the resistivity of the conductor. Accordingly, in order to control the amount of heat exerted to a unit area of glass to be constant irrespective of the shape of the glass, electric conductors are required which have various resistivities and to which metallic terminals can be soldered.
Heretofore, as a method for forming such conductors, a method has been known as disclosed in JP-B-57-41763 in which silver powder, low-melting glass frit and an organic varnish are mixed to obtain a paste, which is then fired to a glass plate, and then copper is electroplated to adjust the resistivity.
However, the electroplating treatment has had problems with respect to the operation efficiency and costs, since the number of process steps thereby increases, although desired levels of the resistivity and bond strength with the metallic terminals can be obtained.
Further, it has been proposed to incorporate a metal oxide or a metal having high resistivity such as Ni, Al, Sn, Pb, Pt or Pb as a resistivity-controlling agent to the above mentioned paste, without carrying out such electroplating.
However, when such a resistivity-controlling agent is used, there has been a drawback that the bond strength to the metallic terminals can not be adequately increased. The reason is considered to be such that when the above mentioned metal or its oxide is added as a resistivity-adjusting agent, sintering of silver particles will be hindered, and the sintered structure of silver will be coarse, and accordingly, when metal terminals are soldered thereto, the solder tends to erode the grain boundaries of silver, and silver tends to melt in the solder, whereby the silver structure will be broken, thus leading to so-called solder-erosion defects, whereby the bond strength is believed to deteriorate. Even if the solder-erosion phenomenon does not occur, the silver structure breakage is still likely to occur, since the sintered structure of silver is coarse, whereby the bond strength tends to be poor.
It has been proposed to incorporate an organic rhodium compound in order to overcome such a drawback (JP-B-57-41763). However, such a proposal has a drawback from the viewpoint of costs, since the organic rhodium compound is expensive.